The present invention relates to optical display apparatus, and in particular to optical display apparatus employing fiber optic faceplates in conjunction with conventional high gain or holographic diffusing screens or direct view cathode ray tubes (CRTs).
Fiber optic faceplates have been used in the past in conjunction with holographic diffusion screens. A particular example is disclosed in U.S. Pat. No. 4,586,781, assigned to the assignee of this application and incorporated herein by this reference. However, a field lens is typically added after the faceplate to direct the diffused light to the exit pupil, as shown in FIGS. 1A and 1B. Here, the diffusion screen 20 with its holographic film 22 (e.g., a dichromated gelatin film) is assembled with the fiber optic faceplate 24 and the convex field lens 26. The field lens 26 with its curved surface 27 provides a greater probability of sun ball reflections making their way back to the exit pupil and washing out a portion of the screen. The sun ball reflections may occur with the curved surface 27 of the lens 26 facing the exit pupil (FIG. 1A) or with the surface 27 facing the faceplate 24 (FIG. 1B). The only way to minimize the sun ball problem is to eliminate any curved surface after the screen 24.
Fiber optic faceplates have also been used in the past with direct view CRTs to enhance the contrast. However, since the fibers are all parallel to each other in an conventional fiber optic faceplate, the numerical aperture ("NA") must be kept purposely high in order that the viewer can see all of the CRT from his viewing position. This is illustrated in FIG. 2, where the faceplate 26' is assembled with the CRT 30, with the respective fans 35-37 of rays exiting points 31-33 on the exterior facing surface of the faceplate 26'. The higher the numerical aperture, however, the more ambient light from the environment around the exit pupil can transmit through the faceplate and scatter off the phosphor surface 30A of the CRT 30, thereby reducing contrast.
It is therefore an object of the invention to provide a fiber optic faceplate for use in optical apparatus which absorbs light from ambient surroundings outside the exit pupil of the apparatus and results in improved contrast, and without increasing the probability of sun ball reflections interfering with viewing of the image produced by the optical apparatus.